Flexible apparatus and method to enhance capacitive force sensing

ABSTRACT

A flexible apparatus and method to enhance capacitive force sensing is disclosed. In one embodiment, a force measuring device includes a sensor capacitor having a fixed surface and a movable surface substantially parallel to the fixed surface, at least one spring assembly (e.g., may deflect longitudinally and/or perpendicularly to a direction of the force) positioned between the fixed surface and the movable surface (e.g., the spring assembly may alter in height in response to a force applied perpendicular to the movable surface and to cause a change in the gap between the fixed surface and the movable surface), and a circuit to generate a measurement of the force based on an algorithm that considers a change in a capacitance of the sensor capacitor. A reference capacitor may adjust the measurement of the applied force based on one or more environmental conditions.

CLAIM OF PRIORITY

This application claims priority from the provisional application60/461,528 filed on Apr. 9, 2003, the non-provisional application Ser.No. 10/823,518 filed on Apr. 9, 2004, the non-provisional applicationSer. No. 11/237,060 filed on Sep. 28, 2005, and the non-provisionalapplication Ser. No. 11/237,353 filed on Sep. 28, 2005.

FIELD OF TECHNOLOGY

This disclosure relates generally to the technical fields of measuringdevices and, in one embodiment, to gap-change sensing through capacitivetechniques.

BACKGROUND

A load cell may be a device (e.g., a transducer) that converts a forceto a differential signal (e.g., a differential electric signal). Theload cell may be used for a variety of industrial applications (e.g., ascale, a truck weigh station, a tension measuring system, a forcemeasurement system, a load measurement system, etc.) The load cell maybe created using a strain gauge. The strain gauge can be used to measuredeformation (e.g., strain) of an object. The strain gauge may include aflexible backing which supports a metallic foil pattern etched onto theflexible backing. As the object is deformed, the metallic foil patternis deformed, causing its electrical resistance to change.

The strain gauge can be connected with other strain gauges to form aload cell in a Wheatstone-bridge configuration (e.g., constructed fromfour strain gauges, one of which has an unknown value, one of which isvariable, and two of which are fixed and equal, connected as the sidesof a square). When an input voltage is applied to the load cell in theWheatstone-bridge configuration, an output may become a voltageproportional to the force on the load cell. The output may requireamplification (e.g., 125×) by an amplifier before it can be read by auser (e.g., because the raw output of the Wheatstone-bridgeconfiguration may only be a few milli-volts). In addition, the load cellin the Wheatstone-bridge configuration may consume a significant amountof power when in operation (e.g., in milli-watts of power).

Manufacturing the load cell in the Wheatstone-bridge configuration mayinvolve a series of operations (e.g., precision machining, attachingstrain gauges, match strain gauges, environmental protection techniques,and/or temperature compensation in signal conditioning circuitry, etc.).These operations may add complexity that may deliver a yield rate ofonly 60%, and may allow a particular design of the load cell to onlyoperate for a limited range (e.g., between 10-5,000 lbs.) ofmeasurement. In addition, constraints of the Wheatstone-bridgeconfiguration may permit only a limited number of form factors (e.g., ans-type form factor and/or a single point form factor, etc.) to achievedesired properties of the load cell. The complexity of variousoperations to manufacture and use load cell may drive costs up (e.g.,hundreds and thousands of dollars) for many industrial applications.

Conventional capacitive force sensing devices suffer from severalconstraints of the springs which are used in such devices. Some of theseconstraints are relaxation and/or creep, hysteresis, set, and off-axisloading. Particularly, hysteresis is a limitation inherent to the use ofvarious springs (e.g, lagging of an effect behind its cause). When thereis a difference in spring deflection at the same applied load—duringloading and/or unloading—the spring may have hysteresis. Hysteresiscould result from set, creep, relaxation and/or friction. Hysteresis maylimit the usefulness of a capacitive force sensing device. Specifically,the spring may consistently and repeatedly return to its originalposition as the load is applied and/or removed. Failure to do so maycause erroneous readings.

An off-axis loading may occur when the direction of an applied load isnot along a normal axis of a sensor. The off-axis loading can cause thesurfaces to become non-parallel and/or can significantly impact variousmeasurements. Many traditional springs such as helical springs orelastomeric springs (made from polymers, e.g., rubber or plastic) maysuffer from many of the above constraints and consequently may not besuitable for high precision applications.

SUMMARY

A flexible apparatus and method to enhance capacitive force sensing isdisclosed. In one aspect, a force measuring device includes a sensorcapacitor having a fixed surface and a movable surface substantiallyparallel to the fixed surface, at least one spring assembly positionedbetween the fixed surface and the movable surface (e.g., may alter inheight in response to a force applied perpendicular to the movablesurface and to cause a change in a gap between the fixed surface and themovable surface), and a circuit to generate a measurement of the forcebased on an algorithm that considers a change in a capacitance of thesensor capacitor.

The force measuring device may include a reference capacitor to adjustthe measurement based on one or more environmental conditions. Ashielding spacer may be placed between the reference capacitor and abottom layer to minimize an effect of a stray capacitance affecting themeasurement. One or more spring assemblies may deflect longitudinallyand/or perpendicularly to a direction of the force such that aperpendicular deflection does not contact the movable surface and thefixed surface.

The spring assemblies may be formed by a conical washer having an insideedge of the conical washer that is wider than an outside edge of theconical washer. The conical washer may be stacked with other conicalwashers to form the at least one spring assembly. The fixed surfaceand/or the movable surface may be painted on any number ofnon-conductive printed circuit boards.

In another aspect, a force measuring device includes a sensor capacitorhaving a fixed surface and a movable surface substantially parallel tothe fixed surface, a fixed layer perpendicular to the movable surface,at least one spring assembly positioned between the movable surfaceand/or the fixed layer to alter in height in response to a force appliedparallel to the movable surface (e.g., and to cause a change in anoverlap area between the fixed surface and the movable surface), and acircuit to determine a measurement based on an algorithm that considersa change in capacitance when the overlap area changes. A referencecapacitor may be integrated in the force measuring device to adjustbased on one or more environmental conditions between the fixed surfaceand another fixed surface.

In yet another aspect, a method to measure force includes positioning atleast one spring assembly between a fixed surface and a movable surface,applying a force (e.g., a load, a stress, etc.) perpendicular to themovable surface to cause a change in the height of the at least onespring assembly and to cause a change in a gap between the fixed surfaceand the movable surface, and automatically generating a measurement of aforce based on an algorithm that considers a change in a capacitancebetween the fixed surface and the movable surface. The measurement ofthe force may be adjusted based on a change in a reference capacitancethat is affected primarily because of one or more environmentalconditions.

In a further aspect, a system (e.g., and/or method) to measure force mayinclude positioning an elastic device between a movable surface and afixed surface perpendicular to the movable surface, causing the elasticdevice to change form based on a force applied adjacent to the movablesurface, and automatically generating a measurement of the force basedon a change in an overlap area between a fixed surface and the movablesurface. In addition the system may include forming the referencecapacitor by substantially parallel plates of the fixed surface and areference surface, and adjusting the measurement based on a change incapacitance of a reference capacitor whose capacitance changes primarilybecause of one or more environmental conditions. The methods, systems,and apparatuses disclosed herein may be implemented in any means forachieving various aspects, and may be executed in a form of amachine-readable medium embodying a set of instructions that, whenexecuted by a machine, cause the machine to perform any of theoperations disclosed herein. Other features will be apparent from theaccompanying drawings and from the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments are illustrated by way of example and not limitationin the figures of the accompanying drawings, in which like referencesindicate similar elements and in which:

FIG. 1 is a cross-sectional view of a metal conical washer positionedbetween a fixed surface and a movable surface and exhibiting adeflection in response to an applied force, according to one embodiment.

FIG. 2 is a cross-sectional view of two metal conical washers positionedback to back between the fixed surface and the movable surface of FIG.1, according to one embodiment.

FIG. 3 is a cross-sectional view of multiple metal conical washerspositioned back to back between the fixed surface and the movablesurface, according to one embodiment.

FIG. 4 is a cross-sectional view of multiple sets of multiple metalconical washers positioned back to back between the fixed surface andthe movable surface, according to one embodiment.

FIG. 5 is a three-dimensional view of a stacked gap-change sensingdevice having a sensor capacitor and a reference capacitor, according toone embodiment.

FIGS. 6A-6G are exploded views of the stacked device of FIG. 1,according to one embodiment.

FIG. 7 is an area-sensing device formed by two substantially parallelsurfaces and a spring assembly positioned between the movable surfaceand a fixed layer, according to one embodiment.

FIG. 8 is a multi-depth area-sensing device, according to oneembodiment.

FIG. 9 is a process view to automatic generate a measurement based on achange in a gap and/or a change in an overlap area between a fixedsurface and a movable surface, according to one embodiment.

FIG. 10 is a three-dimensional view of a carved material that can beused to encompass the sensor capacitor and the reference capacitor inthe boxed device, according to one embodiment.

FIG. 11 is a three-dimensional view of multiple layers of a materialthat can be used to encompass the sensor capacitor and the referencecapacitor in a boxed device, according to one embodiment.

FIG. 12 is a process view to automatically generate a measurement of aforce based on an algorithm that considers a change in a capacitancebetween a fixed surface and a movable surface, according to oneembodiment. At operation 1002, at least one spring assembly (e.g., the

FIG. 13 is a process view to apply a load perpendicular to a movablesurface to cause a change in a height of the at least one springassembly and to cause a change in a gap between a fixed surface and themovable surface, according to one embodiment.

Other features of the present embodiments will be apparent from theaccompanying drawings and from the detailed description that follows.

DETAILED DESCRIPTION

Example embodiments, as described below, may be used to providehigh-accuracy, low-cost, force sensing devices (e.g., load sensors,pressure sensors, etc.). It will be appreciated that the variousembodiments discussed herein may/may not be the same embodiment, and maybe grouped into various other embodiments not explicitly disclosedherein. In the following description, for purposes of explanation,numerous specific details are set forth in order to provide a thoroughunderstanding of the various embodiments. It will be evident, however,to one skilled in the art that the various embodiments may be practicedwithout these specific details.

A spring assembly which overcomes the problems of relaxation, creep,hysteresis, set, and/or off-axis loading is disclosed in one embodiment.The spring assembly in its various embodiments has the property thatwhen a force is applied to the spring assembly, the spring assemblydeflects both longitudinally (e.g., along a direction of an appliedforce) and perpendicularly to a direction of the applied force. However,at the points where the spring assembly contacts other surfaces and/orlayers, a perpendicular deflection is negligible which reduces theproblem of friction and, therefore, hysteresis.

The various embodiments of the spring assembly may be used in differenttypes of force measuring devices (e.g., a gap-change sensing device, anarea-change sensing device, etc.). The spring assembly may include aconical metal washer. The metal conical washer may provide severalsubstantial advantages. The metal conical washer may have a large base(e.g., 3×) compared to its height combined with a large flat top surfacewhich makes it unlikely that the applied force will cause the movablesurface to suffer off-axis loading thus becoming non-parallel. Further,metals may be less susceptible to set and creep than other materials.

In another embodiment of the spring assembly, the spring assembly mayinclude two conical washers placed back to back in such a way that thetop and/or bottom surfaces are wide, but not as wide as the middle. Inanother embodiment, the spring assembly may include multiple pairs ofconical washers placed back to back. In yet another embodiment of thespring assembly, the spring assembly includes multiple sets of conicalmetal washers placed base to base, each set including at least oneconical metal washer.

The various embodiments of the spring assembly may be used in differenttypes of force measuring devices (e.g., a gap-change sensing device, anarea-change sensing device, etc.). In a gap-change sensing device, thespring assembly can be positioned between a fixed surface and a movablesurface which is substantially parallel to the fixed surface. When aforce is applied perpendicular to the movable surface, the height of thespring assembly may be changed and this may cause change in the gapbetween the fixed surface and the movable surface. The change in the gapbetween the fixed surface and the movable surface may cause a change inthe capacitance between the fixed surface and the movable surface, whichcan algorithmically be measured as a force.

In an area-change sensing device, a sensor capacitor may have a fixedsurface and a movable surface substantially parallel to the fixedsurface, a fixed layer perpendicular to the fixed surface, and at leastone spring assembly positioned between the movable surface and the fixedlayer to alter in height in response to a force applied adjacent to themovable surface, and to cause a change in an overlap area between thefixed surface and the movable surface, and a circuit to determine ameasurement based on an algorithm that considers a change in capacitancewhen the overlap area changes.

A spring assembly as disclosed in the various embodiments herein canovercome the problems of relaxation, creep, hysteresis, set, and/oroff-axis loading which are prevalent in conventional springs used indifferent force measuring devices through the use of flexible deviceshaving elastic qualities (e.g., spring assemblies, devices 100, 200,300, 400 of FIGS. 1-4, etc.). The spring assemblies in their variousembodiments may have the property that when a force is applied to thespring assembly, the spring assembly may deflect both longitudinally(along the direction of the applied force) and perpendicularly to thedirection of the applied force. However, at the points where the springassembly contacts other surfaces and/or layers, the perpendiculardeflection may be negligible which reduces the problem of friction and,therefore, hysteresis.

A few embodiments of spring assembly have been shown in FIGS. 1-4 by wayof illustration. The various embodiments of the spring assembly may beused in different types of force measuring devices (including, e.g., agap-change sensing device, an area-change sensing device, etc.).

FIG. 1 is a cross-sectional view of a device 100, with a conical washer140 positioned between a fixed surface 170 and a movable surface 110,and exhibiting a deflection in response to an applied force 105,according to one embodiment. The conical washer 140 may have an insideedge that is wider than an outside edge, and may be made of metal (e.g.,metals may be less susceptible to set and creep than other materials).In alternate embodiments, the conical washer 140 may be created from asynthetic material (e.g., a polymer based material). The conical washer140 may deflect both longitudinally 120 (along the axis) andperpendicularly 160 (perpendicular to the axis) to the direction ofunknown force 105. When the force 105 is applied to the conical metalwasher 140, the movable surface 110 shifts to the position 150.

At the points where the conical metal washer is in contact with othersurfaces and/or layers (e.g., the movable surface 110), a perpendiculardeflection (e.g., perpendicular to the direction of the force 105) maybe negligible. This may reduce friction and, therefore, hysteresis. Thefixed surface 170 and the movable surface 110 may be painted (e.g.,sputtered, coated) on multiple non-conductive printed circuit boards(e.g., the printed circuit boards 502,506,510 of FIG. 5). The conicalmetal washer 140 may have a large base compared to its height. Inaddition, a large flat top surface may make it unlikely that the appliedforce will cause off-axis loading.

FIG. 2 is a cross-sectional view of a device 200, with two metal conicalwashers positioned back to back between the fixed surface 170 and themovable surface 110, according to one embodiment. A first conical washer230 and a second conical washer 260 may be placed back to back in such away that the top and bottom surfaces are wide, but not as wide as themiddle. As a force 105 is applied against the movable surface 110, itmay cause a longitudinal deflection 220 in the device 200, andperpendicular deflections 270 and 280 in the conical washers 230 and260. However, at the points where the conical washer 230 contacts themovable surface 110 and where the conical washer 260 contacts the fixedsurface 170, perpendicular deflections 240 and 250 are negligible, whichmay reduce the problem of friction and therefore, hysteresis. Theconical washers 230 and 260 may be bonded together using an adhesiveand/or glue in one embodiment. In alternate embodiments, the conicalwashers 230 and 260 may be welded together.

FIG. 3 is a cross-sectional view of a device 300, with multiple metalconical washers positioned back to back between the fixed surface 170and the movable surface 110, according to one embodiment. As the force105, also shown in FIG. 1, is applied against the movable surface 110,it causes longitudinal deflection 320 in the spring assembly, andperpendicular deflections in conical washers 330, 340, 350, and 360.However, at the points where the conical washer 330 contacts the movablesurface 110, where the conical washer 360 contacts the fixed surface170, and also where the conical washer 340 contacts conical washer 350,perpendicular deflections may be negligible. The

FIG. 4 is a cross-sectional view of a device 400, with multiple sets ofmultiple metal conical washers positioned back to back between the fixedsurface and the movable surface, according to one embodiment. FIG. 4illustrates a device 400 in which multiple sets (e.g., one set may havetwo washers) of conical washers are placed base to base (e.g., back toback), each set including at least one conical metal washer. As theforce 105, also shown in FIG. 1, is applied against the movable surface110, it may cause longitudinal deflection 420 in the device 400, andperpendicular deflections in all the conical washers, similar to theperpendicular deflections shown in FIGS. 2 and 3. The device 300 of FIG.3 and the device 400 of FIG. 4 illustrate different configurations ofthe device 200 of FIG. 2 that may be employed to provide furtheradvantages in various applications (e.g., higher load measurementcapacity, lesser likelihood of off-axis loading).

In a gap-change sensing device, a spring assembly (e.g., the assembly ofconical washers 330, 340, 350, and 360 of FIG. 3) may be positionedbetween a fixed surface (e.g., the fixed surface 170 of FIG. 1) and amovable surface (e.g., the movable surface 110 of FIG. 1) that issubstantially parallel to the fixed surface. When a force is appliedperpendicular to the movable surface, it causes change in the gapbetween the fixed surface and the movable surface. The change in the gapbetween the fixed surface and the movable surface may cause a change inthe capacitance between the fixed surface and the movable surface. Agap-change sensing device may generate a measurement based on the changein capacitance of a sensor capacitor resulting from a change in a gapbetween a fixed surface and a movable surface. A reference capacitor maybe used to adjust the measurement based on at least one environmentalcondition.

FIG. 5 is a three-dimensional view of a stacked device 550 having asensor capacitor (e.g. formed by the fixed surface 170 and the movablesurface 110 of FIG. 1) and a reference capacitor (e.g., formed by thesurface 622 of FIG. 6C and the surface 628 of FIG. 6E), according to oneembodiment. The stacked device 550 of FIG. 5 includes a top layer 500, aprinted circuit board 502, a device 504 (e.g., the devices100,200,300,400), a printed circuit board 506, a spacer 508, a printedcircuit board 510, a shielding spacer 512, and a bottom layer 514. Acable 516 (e.g., an interface cable) may connect the stacked device 550to a data processing system. In addition, a force 518 (e.g., a load, aweight, a pressure, etc.) may be applied to the top layer 500. Thevarious components of the stacked device 550 are best understood withreference to FIG. 6A-6G.

FIGS. 6A-6G are exploded views of the stacked device 550 of FIG. 5. FIG.6A illustrates the top layer 500 and the printed circuit board 502. Thetop layer 500 may be created from a material such as aluminum, steel,and/or a plastic, etc. The printed circuit board 502 includes a surface616. The surface 616 may be painted (e.g., sputtered, coated, etc.) onthe printed circuit board 502. The printed circuit board 502 may becoupled (e.g., screwed onto, bonded, etched, glued, affixed, etc.) tothe top layer 500 as illustrated in FIG. 6A so that when the force 518(e.g., as illustrated in FIG. 5) is applied to the top layer 500, theheight of the spring assembly 504 (e.g., as illustrated in FIG. 5) isreduced, resulting in change in the gap between the surface 616 and asurface 620 separated by the spring assembly 504 as illustrated in FIG.6B.

FIG. 6C is a view of the printed circuit board 506 (e.g., anon-conductive material). In the embodiment illustrated in FIG. 6C, asurface 620 (e.g., a conductive surface) is painted (e.g., coated,sputtered, etc.) on the printed circuit board 506 on one side. Inaddition, a surface 622 may be painted on the other side of the printedcircuit board 506 as illustrated in FIG. 6C. The surface 616 may bepainted (e.g., sputtered, coated, etc.) on the printed circuit board506. The change in the gap between the surface 616 and the surface 620may cause a change in capacitance of a sensor capacitor (e.g., thesensor capacitor formed by the surface 616 and the surface 620 separatedby the spring assembly 504.

In one embodiment, the surface 616 and the surface 620 are substantiallyparallel to each other and have the same physical area and/or thickness.A change in capacitance of the sensor capacitor may be inverselyproportional to the change in the distance between the surface 616 andthe surface 620 in one embodiment.

The spring assembly 504 of FIG. 6C may be coated with an insulatingmaterial at the ends where it comes in contact with the fixed surface620 and the movable surface 616 (e.g., to avoid a short circuit). In oneembodiment, the spring assembly 504 may be created from a conductivesynthetic material rather than solely one or more metals. The springassembly 504 may create a gap between the surface 616 and the surface620. The gap can be filled with air or any other gas (e.g., an inertgas).

The surface 622 as illustrated in FIG. 6C and the surface 628 asillustrated in FIG. 6E may be separated by the spacer 508 as illustratedin FIG. 6D. The surface 622 and the surface 628 may form a referencecapacitor according to one embodiment. Since the surface 622 and thesurface 628 may not alter positions with respect to each other when theforce 518 is applied to the top layer 500, their capacitance may notchange (e.g., capacitance is calculated as “capacitance=(dielectricconstant multiplied by area of overlap) divided by (distance betweensurfaces)”) in response to the applied force 518.

As such, the reference capacitor formed by the surface 622 and thesurface 628 may experience a change in capacitance only forenvironmental factors (e.g., humidity in a gap between the first surfaceand the second surface, a temperature of the stacked device 550, and anair pressure of an environment surrounding the stacked device 550,etc.). Therefore, the effect of these environmental conditions can beremoved from a measurement of a change in capacitance of the sensorcapacitor (e.g. formed by the surface 616 and the surface 620) when theforce 518 is applied to the stacked device 550 to more accuratelydetermine a change in capacitance of the sensor capacitor.

A processing module 624 as illustrated in FIG. 6E of the stacked device550 may be used to generate a measurement based on a change in adistance between the surface 616 of FIG. 6A and the surface 620 of FIG.6C (e.g., through coupling the stacked device 550 through a connector624 of FIG. 6E with the cable 512 of FIG. 5). In addition, theprocessing module 624 may generate a measurement of the sensor capacitorafter removing an effect of the environmental condition from acapacitance of the sensor capacitor (e.g., by subtracting the changes inthe reference capacitor, which may be only affected by environmentalconditions).

The shielding spacer 512 as illustrated in FIG. 6F may separate theprinted circuit board 510 from a bottom layer 514 (e.g., to minimize aneffect of a stray capacitance affecting the measurement). The bottomlayer 514 is illustrated in FIG. 6G. The various components illustratedin FIGS. 6A-6G may physically connect to each other to form the stackeddevice 550 in one embodiment (e.g., in alternate embodiments the variouscomponents may be screwed together, welded together, bound together,etc.).

The spring assembly 504 of the stacked device 550 of FIG. 5 in differentembodiments may include one or more metal conical washers. According toone embodiment, the spring assembly 504 of the stacked device 550 mayinclude one conical washer, as illustrated in FIG. 1. According toanother embodiment, the spring assembly 504 of the stacked device 550may include a pair of conical washers, as illustrated in FIG. 2.According to another embodiment, the spring assembly 504 of the stackeddevice 550 may include multiple pairs of conical washers stacked on topof each other, as illustrated in FIG. 3. According to yet anotherembodiment, the spring assembly 504 of the stacked device 550 mayinclude multiple sets of conical washers, each set including at leastone conical washer, as illustrated in FIG. 4.

FIG. 7 is an area-sensing device 750 formed by two substantiallyparallel surfaces and a spring assembly positioned between the movablesurface and a fixed layer, according to one embodiment. Device 750includes a top layer 702 (e.g., a conductive and/or non-conductivesubstrate) and a bottom layer 704 (e.g., a conductive and/ornon-conductive substrate), according to one embodiment. A force 700 isapplied to the top layer 702 in FIG. 7. The top layer 702 includes amovable surface 706 perpendicular to the top layer 702. The bottom layer704 includes a surface 708 and a surface 710, both the surfacesperpendicular to the bottom layer 704.

The movable surface 706 is substantially perpendicular to the fixedlayer 704, but is not directly in contact with the fixed layer, thedevice 504 being positioned between the movable surface 706 and thefixed layer 704 (e.g., illustrated as encompassed by a rectangularnon-conductive material that can flex, such as a polymer basedmaterial). The surface 706 and the surface 708 (e.g., the surface 706and the surface 708 may be substantially parallel to each other) form asensor capacitor 714 (e.g., the sensor capacitor 714 may be a variablecapacitor formed because two conductive surfaces/plates are separatedand/or insulated from each other by an air dielectric between thesurface 706 and the surface 708) in an area that overlaps the surface706 and the surface 708. The surface 706 may be movable relative to thesurface 708 in one embodiment. In addition, a reference capacitor 712 isformed between the surface 708 and the surface 710 (e.g., a referencesurface). The surface 710 may be substantially parallel to the surface706 and/or with the surface 708 in one embodiment. In addition, thesurface 710 may be electrically coupled to the surface 706 and/or thesurface 708. Since the surface 708 and the surface 710 may not alterpositions with respect to each other when the force 700 is applied tothe top layer 710, their capacitance may not change.

The spring assembly 504 of the area-sensing device 750 of FIG. 7 indifferent embodiments may include one or more metal conical washers.According to one embodiment, the spring assembly 504 of the area-sensingdevice 750 may include one conical washer, as illustrated in FIG. 1.According to another embodiment, the spring assembly 504 of thearea-sensing device 750 may include a pair of conical washers, asillustrated in FIG. 2. According to another embodiment, the springassembly 504 of the area-sensing device 750 may include multiple pairsof conical washers stacked on top of each other, as illustrated in FIG.3. According to yet another embodiment, the spring assembly 504 of thearea-sensing device 750 may include multiple sets of conical washers,each set including at least one conical washer, as illustrated in FIG. 4

FIG. 8 is a multi-depth device 850 according to one embodiment. In FIG.8, a top layer 702, a middle layer 704, and a bottom layer 814 areillustrated. The top layer 702 includes a plate 706 (e.g., a conductivesurface). The plate 706 may be electrically separated from the top layer702 by application of an insulating material between an area ofaffixation between the top layer 702 and the plate 706. A force 700 maybe applied to the top layer 702 and the plate 706 to cause the plate 706to deflect (e.g., move inward once a load and/or force 700 is applied tothe top layer 702 as illustrated in FIG. 8). The movable surface 706 issubstantially perpendicular to the fixed layer 704, but is not directlyin contact with the fixed layer, the device 504 being positioned betweenthe movable surface 702 and the fixed layer 704.

The middle layer 704 includes a plate 708 and the plate 810. In oneembodiment, the middle layer 804 may include two separate layers bondedtogether each having either the plate 708 or the plate 810. The bottomlayer 814 includes a plate 816. In one embodiment, there may be ashielding spacer (e.g., not shown, but the shielding spacer may be anytype of spacer) between the reference capacitor (e.g., formed by theplate 810 and the plate 816) and a bottom of the housing (e.g., thebottom layer 814) to minimize an effect of a stray capacitance affectingthe measurement (e.g., a height of the shielding spacer may be at leastten times larger than a plate spacer between plates of the referencecapacitor and between plates of the sensor capacitor in one embodimentto minimize the stray capacitance). The plate 806 and the plate 808 mayform a sensor capacitor (e.g., as formed by the fixed surface 170 andthe movable surface 110 of FIG. 1). Similarly, the plate 810 and theplate 816 may form a reference capacitor (e.g., as formed by the plate810 and the plate 816).

A spacer 811 may be used to physically separate the top layer 802 fromthe middle layer 804. In one embodiment, the spring assembly 504 (e.g.,conical back to back springs) may be placed between (e.g., in the outerperiphery between) the top plate 702 of FIG. 8 and the housing 811 ofFIG. 8. A spacer 812 may be used to physically separate the middle layer804 from the bottom layer 814. The multi-depth device 850 may be easierto manufacture according to one embodiment because of modularity of itsdesign (e.g., various manufacturing techniques can be used to scale themulti-depth device 850 with a minimum number of sub-assemblies) in thatvarious sub-assemblies may each include only one surface (e.g., the toplayer 802, the middle layer 804, and the bottom layer 816 may includeonly one plate).

FIG. 9 is a process view to automatically generate a measurement basedon a change in a gap and/or a change in an overlap area between a fixedsurface and a movable surface, according to one embodiment. FIG. 9 is aprocess view of measuring a force 900, according to one embodiment. InFIG. 9, a force 900 may be applied to a sensor 902 (e.g., the appliedforce 518 of FIG. 5, or the applied force 700 of FIG. 7), according toone embodiment. An electronic circuitry (e.g., a software and/orhardware code) may apply an algorithm to measure a change in a distancebetween the surface 616 and the surface 620 forming the sensor capacitoras illustrated in FIG. 6A and FIG. 6C (e.g., the sensor 902 may includethe spring assembly 504 of FIG. 5 and/or any one or more of the devices100, 200, 300, and 400 of FIGS. 1-4) when the force 518 of FIG. 5 isapplied to a device (e.g., the stacked device 550). In an alternateembodiment, a change in area between the surfaces may be consideredrather than a change in the gap (e.g., the change in an overlap areabetween the surface 706 and the surface 708 forming the sensor capacitoras illustrated in FIG. 7).

Next, a change in capacitance 906 may be calculated based on the changein the gap between the surfaces forming the sensor capacitor or changein the overlap area between the surfaces forming the sensor capacitor.The change in capacitance 906, a change in a voltage 908, and/or achange in a frequency 910 may also be calculated to generate ameasurement (e.g., an estimation of the force 900 applied to the sensor902). The change in capacitance 906 data, the change in voltage 908data, and/or the change in frequency data 910 may be provided to adigitizer module 912 (e.g., an analog-to-digital converter). Finally,the digitizer module 912 may work with a processing module 914 (e.g., amicroprocessor which may be integrated in the processing module 224) toconvert the change in capacitance 906 data, the change in voltage 908data, and/or the change in frequency data 910 to a measurement reading916.

FIG. 10 is a three-dimensional view of a carved material that can beused to encompass (e.g., provide a housing to) the sensor capacitor(e.g., the sensor capacitor 714 as illustrated in FIG. 7 and thereference capacitor (e.g., the reference capacitor 712 illustrated inFIG. 7) in a boxed device, according to one embodiment. In FIG. 10,single block (e.g., steel) is used to form a bottom cup 1014. In oneembodiment, the bottom cup 1014 in FIG. 10 replaces the bottom layer ofa boxed device, and encompasses the various structures (e.g., capacitivesurfaces/plates, spacers, etc.) between a bottom layer and a top plate.The bottom cup 1014 may be formed from a single piece of metal throughany process (e.g., involving cutting, milling, etching, and/or drilling,etc.) that maintains the structural and/or tensile integrity of thebottom cup 1014. This way, the bottom cup 1014 may be able to withstandlarger amounts of force (e.g., the force 105 of FIG. 1) by channelingthe force downward through the walls of the bottom cup 1014.

FIG. 11 is a three-dimensional view of a multiple layers of a materialthat can be used to encompass the sensor capacitor and the referencecapacitor in a boxed device, according to one embodiment. Particularly,FIG. 11 illustrates a bottom cup 1114 formed with multiple blocks ofmaterial according to one embodiment. A single thin solid metal-blockmay form a bottom layer 1100 as illustrated in FIG. 11. In addition,other layers of the bottom cup 1114 may be formed from layers (e.g., thelayers 1102A-1102N) each laser cut (e.g., laser etched) and/or patterned(e.g., to form the bottom cup 1114 at a cost lower than millingtechniques in a single block as may be required in the bottom cup 1014of FIG. 10). For example, the layers 1102A-1102N may be a standard metalsize and/or shape, thereby reducing the cost of fabricating the bottomcup 1114.

In one embodiment, the bottom cup 1114 in FIG. 11 replaces the bottomlayer of a boxed device, and encompasses the various structures (e.g.,capacitive surfaces/plates, spacers, etc.) between a bottom layer and atop plate. Like in the embodiment of FIG. 10, the bottom cup 1114 ofFIG. 11 may be able to withstand larger amounts of force (e.g., theforce 105 of FIG. 1) by channeling the force downward through the wallsof the bottom cup 1114. Furthermore, the bottom cup 1114 may be lessexpensive to manufacture than the bottom cup 1014 as described in FIG.10 because standard machining techniques may be used to manufacture thebottom cup 1114.

FIG. 12 is a process view to automatically generate a measurement of aforce based on an algorithm that considers a change in a capacitancebetween a fixed surface and a movable surface, according to oneembodiment. At operation 1202, at least one spring assembly (e.g., thearrangements of conical washers as illustrated in FIGS. 1-4) ispositioned between a fixed surface (e.g., the fixed surface 170 ofFIG. 1) and a movable surface (e.g., the movable surface 110 of FIG. 1).At operation 1204, a force (e.g., due to the force 105 of FIG. 1) isapplied perpendicular to the movable surface to cause a change in theheight of the at least one spring assembly and to cause a change in agap between the fixed surface and the movable surface.

At operation 1206, at least one spring assembly (e.g., the springassembly as illustrated in FIG. 2) is deflected longitudinally andperpendicularly to a direction of the force such that a perpendiculardeflection does not contact the movable surface and the fixed surface(e.g., the perpendicular deflection at the points of contact with themovable surface and the fixed surface may be negligible). At operation1208, a measurement of a force may be automatically generated based onan algorithm that considers a change in a capacitance between the fixedsurface and the movable surface. At operation 1210, the measurement ofthe force may be adjusted based on a change in a reference capacitance(e.g., formed by the surface 622 and the surface 628 of FIG. 6), that isaffected primarily because of one or more environmental conditions(e.g., to compensate for changes in the measurement due to environmentalconditions).

FIG. 13 is a process view to apply a force (due to the force 700 of FIG.7) perpendicular to a movable surface (e.g., the movable surface 702 ofFIG. 7) to cause a change in a height of the at least one springassembly (e.g., the spring assembly as illustrated in FIG. 2) and tocause a change in a gap between a fixed surface and the movable surface,according to one embodiment. At operation 1302, an elastic device (e.g.,the device 300 of FIG. 3) is positioned between a movable surface and afixed surface perpendicular to the movable surface. At operation 1304,the elastic device is caused to change form (e.g., contract) based on aforce applied adjacent to the movable surface. At operation 1306, ameasurement of a force is automatically generated (e.g., by a softwarecode and/or hardware) based on a change in an overlap area between afixed surface and the movable surface. At operation 1308, a referencecapacitor may be formed by substantially parallel plates of the fixedsurface and a reference surface (e.g., as formed by the plate 810 andthe plate 816 of FIG. 8). At operation 1310, a measurement is adjustedbased on a change in capacitance of a reference capacitor whosecapacitance changes because of one or more environmental conditions(e.g., temperature and/or humidity)

Although the present embodiments have been described with reference tospecific example embodiments, it will be evident that variousmodifications and changes may be made to these embodiments withoutdeparting from the broader spirit and scope of the various embodiments.Accordingly, the specification and drawings are to be regarded in anillustrative rather than a restrictive sense.

1. A force measuring device comprising: a sensor capacitor having afixed surface and a movable surface substantially parallel to the fixedsurface; at least one spring assembly positioned between the fixedsurface and the movable surface; the at least one spring assembly toalter in height in response to a force applied perpendicular to themovable surface and to cause a change in a gap between the fixed surfaceand the movable surface; and a circuit to generate a measurement of theforce based on an algorithm that considers a change in a capacitance ofthe sensor capacitor.
 2. The force measuring device of claim 1 furthercomprising a reference capacitor to adjust the measurement based on oneor more environmental conditions.
 3. The force measuring device of claim2 further comprising a shielding spacer between the reference capacitorand a bottom layer to minimize an effect of a stray capacitanceaffecting the measurement.
 4. The force measuring device of claim 1wherein the at least one spring assembly to deflect longitudinally andperpendicularly to a direction of the applied force such that aperpendicular deflection does not contact the movable surface and thefixed surface.
 5. The force measuring device of claim 1 wherein the atleast one spring assembly comprises a conical washer having an insideedge of the conical washer that is wider than an outside edge of theconical washer.
 6. The force measuring device of claim 5 wherein theconical washer is stacked with other conical washers to form the atleast one spring assembly.
 7. The force measuring device of claim 1wherein the fixed surface and the movable surface are painted on aplurality of non-conductive printed circuit boards.
 8. A force measuringdevice, comprising: a sensor capacitor having a fixed surface and amovable surface substantially parallel to the fixed surface; a fixedlayer perpendicular to the fixed surface; at least one spring assemblypositioned between the movable surface and the fixed layer to alter inheight in response to a force applied adjacent to the movable surface,and to cause a change in an overlap area between the fixed surface andthe movable surface; and a circuit to determine a measurement based onan algorithm that considers a change in capacitance when the overlaparea changes.
 9. The force measuring device of claim 8 furthercomprising a reference capacitor integrated in the force measuringdevice to adjust based on one or more environmental conditions betweenthe fixed surface and another fixed surface.
 10. The force measuringdevice of claim 8 wherein the at least one spring assembly to deflectlongitudinally and perpendicularly to a direction of the force appliedsuch that a perpendicular deflection does not contact the movablesurface and the fixed surface.
 11. The force measuring device of claim10 wherein the at least one spring assembly comprises a conical washerhaving an inside edge of the conical washer that is wider than anoutside edge of the conical washer.
 12. The force measuring device ofclaim 11 wherein the conical washer is stacked with other conicalwashers to form the at least one spring assembly, and wherein the fixedsurface and the movable surface are painted on a plurality ofnon-conductive printed circuit boards.
 13. A method to measure force,comprising: positioning at least one spring assembly between a fixedsurface and a movable surface; applying a force perpendicular to themovable surface to cause a change in the height of the at least onespring assembly and to cause a change in a gap between the fixed surfaceand the movable surface; and automatically generating a measurement of aforce based on an algorithm that considers a change in a capacitancebetween the fixed surface and the movable surface.
 14. The method ofclaim 13 further comprising adjusting the measurement of the force basedon a change in a reference capacitance that is affected primarilybecause of one or more environmental conditions.
 15. The method of claim13 in a form of a machine-readable medium embodying a set ofinstructions that, when executed by a machine, cause the machine toperform the method of claim
 13. 16. The method of claim 13 wherein theat least one spring assembly to deflect longitudinally andperpendicularly to a direction of the force such that a perpendiculardeflection does not contact the movable surface and the fixed surface,wherein the at least one spring assembly comprises a conical washerhaving an inside edge of the conical washer that is wider than anoutside edge of the conical washer, and wherein the conical washer isstacked with other conical washers to form the at least one springassembly.
 17. A system to measure force, which comprises: means forpositioning an elastic device between a movable surface and a fixedsurface perpendicular to the movable surface; means for causing theelastic device to change form based on a force applied adjacent to themovable surface; and means for automatically generating a measurement ofthe force based on a change in an overlap area between a fixed surfaceand the movable surface.
 18. The system of claim 17 further comprisingmeans for forming the reference capacitor by substantially parallelplates of the fixed surface and a reference surface; and means foradjusting the measurement based on a change in capacitance of areference capacitor whose capacitance changes primarily because of oneor more environmental conditions.
 19. The system of claim 17 furthercomprising means for longitudinally and perpendicularly deflecting theelastic device in a direction of the force such that a perpendiculardeflection does not contact the movable surface and the fixed surface.20. The system of claim 19 wherein the elastic device comprises aconical washer having an inside edge of the conical washer that is widerthan an outside edge of the conical washer, and wherein the conicalwasher is stacked with other conical washers to form the elastic device.